The present invention relates to the art of magnetic resonance imaging (MRI). It finds particular application in conjunction with multi-slice acquisitions employing single echo MRI pulse sequences such as field echo (FE) and spin echo (SE) sequences, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Commonly, in MRI, a substantially uniform, temporally constant main magnetic field, B.sub.0, is set up in an examination region in which a subject being imaged is placed. Via magnetic resonance radio frequency (RF) excitation and manipulations, selected magnetic dipoles in the subject which are otherwise aligned with the main magnetic field are tipped (via RF pulses) into a plane transverse to the main magnetic field such that they precess or resonate. In turn, the resonating dipoles are allowed to decay or realign with the main magnetic field thereby inducing magnetic resonance echoes. The various echoes making up the MRI signal are encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI apparatus is collected into a matrix commonly known as k-space. Typically, each echo is sampled a plurality of times to generate a data line or row of data points in k-space. The echo or data lines position in k-space is determined by its gradient encoding. Ultimately, employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space data.
At times, due to non-ideal system performance or in the case of some specific data acquisition strategies, MRI signals are distorted or contaminated in either phase or amplitude leading to data inconsistencies in k-space. One potential inconsistency is that from row to row in k-space, each resonance excitation is not precisely the same amplitude or the same phase. Consequently, the result is degraded image quality. Generally, traditional methods of addressing this problem are designed to minimize those known factors affecting image quality, such as gradient non-linearity, etc. One of these methods is, for example, to over sample and use the additionally generated and collected echoes to provide correction factors which are used to adjust the k-space data and compensate for the data inconsistencies. However, while adding proportionally small amounts of time to longer imaging sequences having multi-echo echo-trains, the introduction of these additional echoes, termed navigator echoes, proportionally lengthens imaging sequences by larger amounts for single echo pulse sequences such as SE and/or FE sequences.
Moreover, commonly the navigator echoes are collected at the end of the imaging pulse sequences. Accordingly, data throughput is slowed. That is to say, processing of the k-space image data is delayed until the navigator echoes are collected and processed to generate the correction factors which will operate on the k-space data. As well, the raw MRI data has to be stored until it can be adjusted. Therefore, the efficient throughput achieved by parallel or pipeline data processing is impeded as the correction factors are not readily available upon collection of the relevant k-space data.
The present invention contemplates a new and improved technique for addressing k-space data inconsistencies which overcomes the above-referenced problems and others.